With the growing awareness of environmental protection worldwide, Sn—Ag—Cu lead-free solder has spread as a solder material for mounting an electronic component on a board.
However, lead solder has been still used for, for example, the joining material of inside of an Insulated Gate Bipolar Transistor (IGBT) power module. This is because a solder material used for inside joining of fixing a semiconductor element to the mounting portion of a supporting board does not have to melt at a reflow soldering temperature when joining the module and a mother board. If the solder material used for inside joining remelts at the reflow soldering temperature, the module is short-circuited. The reflow soldering temperature is generally 240° C. to 260° C. Reflow soldering is also called as secondary soldering.
For this reason, lead-free solder which does not melt at a reflow soldering temperature is required. For example, Patent Literature 1 describes high temperature lead-free solder composed mainly of bismuth as a solder material satisfying such a requirement. The high temperature lead-free solder contains about 2% by weight to 18% by weight of silver, about 98% by weight to 82% by weight of bismuth, and up to about 1000 ppm of at least one of zinc, nickel, germanium and a combination of them. Further, the high temperature lead-free solder has a solidus line of not lower than about 262.5° C. and a liquidus line of not higher than about 400° C.
Meanwhile, for example, Patent Literature 2 describes a conductive adhesive as another joining material used for inside joining. The conductive adhesive is obtained by mixing and dispersing Ag filler and particulate resin component to make a paste with a volatile solvent. The content of Ag filler is 90% at the maximum, and the thermal conductivity of the conductive adhesive is 60 W/(m·K) at the maximum.